The reclaim rubber industry is an old one, founded mainly upon the alkali process in which ground polyisoprene rubber scrap was cooked in aqueous sodium hydroxide (NaOH) at about 190.degree. C. for many hours. Aqueous sodium hydroxide dissolved the residual fiber and any sulfur that diffused to the surface. The concurrent softening of the rubber is attributable chiefly to thermal oxidative main chain scission. The extreme incompatibility of aqueous NaOH with rubber minimizes the participation of NaOH in the scission of polysulfide crosslinks, such as may occur. Moreover, natural rubber vulcanizates could be reclaimed even through the application of heat alone. The alkali process was abandoned with the advent of butadiene-styrene rubber because of "heat hardening" of SBR vulcanizates under these conditions.
Three processes, popularly referred to as the digester process, the heater process, and, the reclaimator process, currently produce essentially all the reclaim rubber (referred to for brevity herein as "reclaim") in the U.S. Scrap rubber in these processes is reclaimed through the application of high temperatures and intense working, that is, the application of severe mechanical shear. The rubber is first ground, defibered, and mixed with depolymerization chemicals. In the heater and reclaimator process, this mixture is heated at high temperatures for prolonged periods followed by the application of intense working. Network destruction occurs mainly through the breaking of bonds in the main polymer chain; although, some sulfide crosslinks are also likely broken. Depolymerization chemicals assist this process, probably through free radical chain transfer reactions. Though oxygen is not deliberately added, enough must be present to enable oxidation processes to occur. The reclaimator process disclosed in U.S. Pat. Nos. 2,415,449; 2,633,602; 2,653,348; 2,653,915; 2,804,651; and 3,051,990 inter alia, differs in that it is carried out in an extruder where both heat and shear are applied simultaneously. The structure of the resulting reclaim is characteristically a branched three-dimensional network of shortened polymer chains. Such reclaim bears little resembalance to the original rubber from which the reclaim was derived. This dissimilarity of reclaim is reflected in low tensile strength, poor abrasion resistance, and poor dynamic properties of cured rubber compounds containing a significant quantity of reclaim. These currently used prior art processes are thermomechanical methods which cause indiscriminate network destruction, most of which is hydrocarbon main chain scission. As a result, conventional reclaim is normally used only in small amounts in most tire rubbers. (see Scrap Tire Disposal, Beckman, J. A. et al, Rubber Chem. Techol. 47 (3), 597 (1974)).
An effort to produce a recyclable reclaim rubber without altering its network structure is disclosed in U.S. Pat. No. 4,046,834 to Lee, T. C. and Millns, W. In one process disclosed therein, shredded coarse crumbs of scrap rubber absorb a fatty acid and are dusted with solid alkali. The rubber is rolled on a mill, comminuted, dispersed in an aqueous mixture and recovered as a sub-20 micron powder. In another process disclosed in the Lee et al patent, the vulcanized rubber crumb is swelled with an organic water-miscible liquid and comminuted to a sub-20 micron particle size in a liquid medium from which a dry reclaim rubber fine powder is obtained. The aforementioned Lee et al process is clearly a particle size reduction process in which NaOH, as stated therein, does not alter the network structure.
It is known that hydroxide ions (OH.sup.-) chemically break polysulfide bonds in simple organic molecules. Though prior art processes have cooked comminuted vulcanized rubber scrap in hot sodium hydroxide the purpose was mainly to remove fiber. To utilize this polysulfide bond-breaking chemistry of OH.sup.- ions effectively, it is necessary to penetrate the heretofore impregnable elastomeric networks in vulcanized scrap with OH.sup.- ions at relatively low temperatures compared to those normally used in reclaim processes. If accomplished, this should yield a devulcanized rubber which, irrespective of what it is termed, is chemically distinguishable from prior art reclaim rubbers. The essential chemical distinction is that in devulcanized rubber, nearly all of the polysulfide crosslinks are selectively broken without significant main chain scission. An accepted definition of devulcanized rubber is that "It requires only that vulcanized rubber lose its elastic properties and become less resistant to compression, stretching, or swelling." ("Reclaim Rubber" by Bobby LaGrone, U.S. Rubber Reclaim Corp., in a presentation at the B. F. Goodrich Company in Akron, Ohio on Apr. 13, 1977). In other words, devulcanized rubber is more plastic than vulcanized rubber from which it is derived. However, I refer to "devulcanized rubber" which in addition has had a substantial proportion of polysulfide crosslinks broken by chemical means with negligible main chain scission. Though selective scission of polysulfide crosslinks has been achieved in other ways, most of these methods are not commercially practical, though they are useful for analytical purposes. For example, LiAlH.sub.4 and C.sub.6 H.sub.5 Li have been used to cleave polysulfide crosslinks when swollen into rubber with an anhydrous organic solvent.
The organic onium salts of nitrogen, phosphorus and sulfur are well known. They are ionized in aqueous solutions to form stable cations. Certain onium salts have provided the basis for phase transfer catalysis in a wide variety of reactions, a recent and comprehensive review of which is contained in Angewandte Chemie, International Edition in English, 16 493-558 (August 1977). Discussed therein are various anion transfer reactions where the onium salt exchanges its original anion for other anions in the aqueous phase. These ion pairs can then enter a water immiscible, organic liquid phase, making it possible to carry out chemistry there with the transported anion, including OH.sup.- ions. Many reactions involving water immiscible solutions of relatively simple organic molecules have been described. However, these mobile, liquid phase systems are quite distinct from the complex, immobile network structure of a scrap rubber particle, where the organic substrate is an insoluble, crosslinked polymer. To my knowledge, there is no teaching of phase transfer catalysis in a rubber vulcanizate for any purpose whatsoever, and no indication that this technology would be useful for the selective scission of polysulfide crosslinks in a rubber vulcanizate.
This invention discloses a phase transfer catalyzed devulcanization process which fills a long-felt need for coping with the twin problems of disposing of used tires responsibly, and, increasing the supply of hydrocarbon-derived raw materials.